Method of integrating a single nanowire into a nanocircuit

ABSTRACT

A non-volatile bistable nano-electromechanical switch is provided for use in memory devices and microprocessors. The switch employs carbon nanotubes as the actuation element. A method has been developed for fabricating nanoswitches having one single-walled carbon nanotube as the actuator. The actuation of two different states can be achieved using the same low voltage for each state.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/228,701, filed on Sep. 9, 2011 entitled, BISTABLE NANOSWITCH which isa divisional of U.S. application Ser. No. 12/594,954, now U.S. Pat. No.8,031,514 issued on Oct. 4, 2011 entitled, BISTABLE NANOSWITCH which isa 371 of International Application No. PCT/US2008/004584, filed on Apr.9, 2008 and claims the priority of U.S. Provisional Application No.60/922,468 filed on Apr. 9, 2007 and entitled BISTABLE NANOSWITCH. Allof the aforementioned applications are hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The invention was made with support from Grant NSF-0425826 from theNational Science Foundation Nanoscale Science and Engineering Center.The United States Government has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to nanotechnology. More specifically, theinvention relates to a non-volatile bistable nanoswitch for anelectrical circuit.

BACKGROUND OF THE INVENTION

Switching devices used in electronic components such as memory chips andmicroprocessors preferably provide high density, non-volatile storage athigh speed, low power consumption, and low cost. The use ofnanotechnology has promised improvements in all of these areas. Forexample, WO 01/03208 describes the use of single-walled carbon nanotubes(SWNT) in crossbar junctions as memory devices. In a nanoscale crossbarmemory device, a nanowire such as a SWNT is suspended over othernanowires at a junction. An electrical current in one or both nanowirescauses either attraction or repulsion of the wires, resulting in thestable closing or opening of an electrical circuit. However, suchdevices are considered difficult to produce reliably and to usecommercially, because of the problems inherent in using directed growthor chemical assembly of the nanowires at the scale required. In otherproposed nanoscale switching devices, such as described in U.S. Pat. No.6,919,592, sheets of nanoelements such as SWNT are used as actuationelements. With such devices, the ratio of the voltage required to switchto one state is on the order of six-fold higher than the voltagerequired to switch to the other state, adding significant complexity tothe operation of such devices. Thus, there remains a need to developnanoscale switching devices that are simple to construct and to operate,and that provide high density, low power non-volatile storage.

SUMMARY OF THE INVENTION

The invention provides a non-volatile bistable nano-electromechanicalswitch employing one or more nanotubes such as SWNT as the actuationelement. In this design the actuation of each of two different states,as further described below, can be achieved at about the same voltage,and at a relatively low voltage, compared to other SWNTelectromechanical devices. The nanoswitch device of the currentinvention can be applied in the fabrication of latches, flip-flops,registers, and other components used, for example, in microprocessorchips for computers and other applications. A nanoswitch deviceaccording to the invention can also be incorporated with existing CMOSprocesses to fabricate non-volatile memory devices, and modifications tothe bistable nanoswitch can result in sensor devices such as radiationdetectors, chemical sensors, thermal sensors, and biological sensors.

One aspect of the invention is a non-volatile, bistable nanoswitchdevice. The device includes as the actuation element a single-walledcarbon nanotube spanning a gap between first and second electricalterminals or contacts exposed on a surface of an insulatingnanosubstrate. The gap between the terminals contains first and secondnanoscale channels. The device further contains a third electricalterminal exposed on the surface of the nanosubstrate. The third terminalis electrically connected to a first conductive layer situated withinthe first channel. The device still further contains a fourth electricalterminal exposed on the surface of the nanosubstrate. The fourthterminal is electrically connected to a conductive layer situated withinthe second channel. When a first voltage is applied between the firstand third terminals, the nanotube moves within the first channel andforms an electrical contact with the conductive layer in the firstchannel. This produces the activation of state 1, in which anon-volatile electrical connection is established between the first andthird terminals. Moreover, application of a second voltage between thesecond and fourth terminals causes the nanotube to form an electricalcontact with the conductive layer in the second channel. This activatesstate 2 by forming a non-volatile electrical connection between thesecond and fourth terminals. The design of the nanoswitch is such thatactivation of state 1 is accompanied by inactivation of state 2, and theactivation of state 2 is accompanied by inactivation of state 1.

Another aspect of the invention is an electrical component that includesone or more nanoswitch devices as described above. In differentembodiments, the electrical component is a latch, a flip-flop, aregister, or a relay.

In another aspect, the invention provides a method of actuating ananoswitch device as described above. The method includes the steps of:providing the nanoswitch device in state 1, having a non-volatileelectrical connection between the first and third terminals; applying avoltage between the second and fourth terminals; and removing thevoltage between the second and fourth terminals. The method establishesa non-volatile electrical connection between the second and fourthterminals, and abolishes the electrical connection between the first andthird terminals.

Still another aspect of the invention is a method of actuating ananoswitch device as described above. The method includes the steps of:providing the nanoswitch device in state 2, having a non-volatileelectrical connection between the second and fourth terminals; applyinga voltage between the first and third terminals; and removing thevoltage between the first and third terminals. The method establishes anon-volatile electrical connection between the first and thirdterminals, and abolishes the electrical connection between the secondand fourth terminals.

Yet another aspect of the invention is a method of making anon-volatile, bistable nanoswitch device. The method includes the stepsof applying an insulating layer onto a substrate surface; applying ametal mask layer onto the insulating layer; applying a photoresist layeronto the metal mask layer; using lithography to fabricate in thephotoresist, metal mask, and insulating layers first, second, third, andfourth nanoscale terminal areas and first and second nanoscale channels;applying a conductive layer onto the terminal areas, the channels, andthe metal mask layer, whereby conductive terminals are formed in theterminal areas and conductive channels within the channels; removing themetal mask layer, leaving conductive layer inside the terminal areas andchannels; and applying one or more actuation nanoelement across theinsulating area and the first and second channels. The nanoelementprovides electrical contact between the conductive layer of the firstand second terminals. The terminals are situated around an insulatingarea containing the channels. The first and second terminals aregenerally situated opposite each other, and the third and fourthterminals are generally situated opposite each other. The third terminalis connected to the first channel and the fourth terminal is connectedto the second channel. The nanoelement actuator lies over the channels,and is capable of movement that establishes electrical contact betweenthe nanoelement and the conductive layer in the channel when voltage isapplied between the conductive layer of a channel and a terminal notconnected to the channel.

Still another aspect of the invention is a method of integrating asingle nanowire, such as a SWNT, into a nanocircuit. The method includesthe steps of: providing a nanosubstrate, a surface of which contains twoconductive terminals separated by an insulating area; adding a liquidsuspension of nanotubes, such as SWNT, to the surface of thenanosubstrate to cover the conductive terminals; performingdielectrophoresis by applying a voltage between the two conductiveterminals; and removing excess SWNT using a stream of gas, such thatonly one SWNT remains in electrical contact with both conductiveterminals.

DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be apparentfrom the following detailed description of the invention, taken inconjunction with the accompanying drawings.

FIG. 1 shows schematic representations of a prior art nanoswitch device(FIG. 1A) and an embodiment of the present invention (FIG. 1B).

FIG. 2 shows a perspective view of a nanoswitch device embodiment inwhich a single nanotube serves as the actuation element.

FIG. 3 illustrates the non-volatility of two states (FIG. 3A, State 1;and FIG. 3B, State 2) in an embodiment of the nanoswitch device of thepresent invention, shown in cross section.

FIG. 4 shows an example of a process for making a nanoswitch device ofthe present invention. In FIG. 4A, the insulating layer and metal masklayer have been etched to prepare conductive terminals and channels. Aconductive layer has been added in FIG. 4B. The unwanted portions of theconductive layer have been removed together with the metal mask layer inFIG. 4C. The actuation element, a single-walled carbon nanotube, hasbeen added in FIG. 4D.

FIG. 5 shows an example of a process for making a nanoswitch device ofthe invention by adding sequential layers to a substrate, performinglithography, and etching.

FIG. 6 illustrates the steps of a process for adding a single-wallednanotube as actuation element to a nanoswitch device.

FIG. 7 shows a series of blunt (FIG. 7A), intermediate (FIG. 7B), andfine-tipped (FIG. 7C) electrodes for use in the dielectrophoreticassembly of a SWNT actuation element.

FIG. 8 shows the effect of performing dielectrophoretic assembly of SWNTactuators using an AC voltage without (FIG. 8A) or with (FIG. 8B) aphase shifter.

FIG. 9A shows a method for fabricating a bistable nanoswitch devicehaving only one SWNT as actuator. FIG. 9B is an enlargement of themicrograph of FIG. 9A.

FIG. 10A shows a theoretical phase diagram of the volatility of thebridge structure as a function of the gap between the electrode and thecarbon nanotube and the width of the electrode. FIG. 10B shows thecalculated upper limit of the actuation voltage as a function of thewidth and gap of the channel.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have developed a bistable nanoscale switching devicecapable of switching between two non-volatile states. The deviceutilizes a nanoelement such as a single SWNT as the actuator. A methodfor fabricating the nanoswitch has been developed in which one SWNT isattached by dielectrophoresis in a crossbar junction configuration thatallows electromechanical forces to stably and repeatedly switch betweentwo alternative electrical connections. The nanoswitch can beincorporated into circuits such as flip-flops, latches, registers, andrelays, and will find application as an element of electrical componentssuch as memory devices and microprocessors.

A comparison between a prior art nanoscale switch device and a deviceaccording to the present invention is shown in FIG. 1. The earlierdevice design shown in FIG. 1A requires a restoring electrode, whosefabrication requires additional steps. The configuration of a deviceaccording to the present invention (FIG. 1B) is simpler, resulting inlower production costs. Further, instead of the patterned sheets ofnanoelements used in many previous devices, the nanoswitch of thepresent invention can use SWNT bundles or even a single SWNT as theactuation element. The result is that less power is consumed in responseto the same voltage applied for switching between two alternativestates. Also, the configuration of the present invention allows the samelow voltage to be used to set either of two states, allowing a 5-foldreduction in voltage compared to prior devices. The dimensions of thedevice are in the nanometer range, and hence result in increased storagedensity compared to larger microscale devices.

Referring now to schematic diagrams of one embodiment of a bistable,non-volatile nanoswitch device of the present invention, a crosssectional view is shown FIG. 1B, and a top perspective view is shown inFIG. 2. Nanoswitch device 5 contains four metallic or conductiveterminals A, B, C and D fabricated on a surface of an insulatingsubstrate by well established nanofabrication and microfabricationmethods. These terminals are also referred to herein as first terminal80, second terminal 85, third terminal 90, and fourth terminal 95. Thefour terminals surround an insulating area on the surface of thesubstrate. The substrate contains base layer 10 for mechanical supportbeneath insulating layer 20. The exact positions of each of theterminals is not critical and can be determined according to thesituation or user preference. However, the first and second terminalsare generally positioned opposite each other, and the third and fourthterminals are generally situated opposite each other. In the embodimentdepicted in FIGS. 1B and 2, the first and second terminals arepositioned about 180 degrees from each other, and the third and fourthterminals are similarly positioned about 180 degrees from each other. Inthis embodiment, a line connecting the first and second terminals isapproximately perpendicular to (at 90 degrees to) a line connecting thethird and fourth terminals. This arrangement permits a crossbar junctionto be formed, as outlined below. Third terminal 90 and fourth terminal95 are extended to form trenches or channels in the insulating areabetween first terminal 80 and second terminal 85. More specifically,third terminal 90 is connected to first channel 40, and fourth terminal95 is connected to second channel 42. As used herein, the first andsecond terminals (80 and 85) may also be referred to as “electrodes”while the first and second channels (40 and 42) may also be referred toas “extension electrodes”. The two channels do not intersect, and do notconnect with either first terminal 80 or second terminal 85. The fourterminals and the two channels are lined with a layer of conductivematerial 30. The conductive layer of the terminals as well as thechannels is preferably below the level of the insulator. SWNT 50electrically connects first terminal 80 with second terminal 85, but inthe open state does not have electrical contact with the conductivelayer in either channel 40 or channel 42. The completely fabricatedsubstrate 100 containing terminals 80, 85, 90, and 95 as well aschannels 40 and 42 can be connected to a circuit, such as circuit 60 orcircuit 70 as shown in FIG. 1B, to form functional nanoswitch device 5.

The geometry described above is easily scalable, such that many morepairs of electrodes (similar to 80 and 85) and trenches (similar to 40and 42) can be fabricated, thereby increasing the number of switches.

Referring now to FIG. 3A, when a potential is applied between terminalsB and C above a critical voltage V_(cr) (also referred to herein as the“first threshold voltage”), due to field effects and the flexibility ofthe SWNT, the SWNT adheres to the extension electrode of terminal B andelectrical continuity is established between terminals B and C. This isdefined as state 1; a cross-sectional view of the state is shown in FIG.3A. During state 1 there is no electrical continuity between terminals Dand C. This critical voltage V_(cr) is a function of the gap between theSWNT and the extension electrode, the width of the extension electrode,and the diameter of the SWNT. Even after the applied voltage between theterminal B and C has been has been turned off the SWNT remains connecteddue to intermolecular forces, until further switching is carried; hencestate 1 is non-volatile.

Referring now to FIG. 3B, when a potential is applied between terminalsD and A above the critical voltage V_(cr) (also referred to herein asthe “second threshold voltage”), the SWNT adheres to the extensionelectrode of terminal D, and electrical continuity is establishedbetween terminals D and A. In doing so, the axial tension of the SWNTincreases enormously, and the SWNT overcomes the intermolecular forcesresponsible for SWNT attachment to the extension electrode of terminalB, thereby interrupting electrical continuity between terminals B and C.This is defined as state 2; a cross sectional view of this state isshown in FIG. 3B. When the potential applied between D and A is removed,the SWNT still adhere to the electrode extension of terminal D, thusmaintaining state 2 until further switching is initiated. The criticalvoltage V_(cr) is a function of the gap between the SWNT and theextension electrode, the width of the extension electrode, and thediameter of the SWNT.

One possible fabrication process is shown schematically in FIG. 4. Astandard E-Beam lithography process (or a similar lithography processsuitable for the nanometer scale, such as UV, immersion or EUVlithography) can be employed to expose the desired patterns onto apositive photoresist spun onto a metal deposited on an insulatingsubstrate. After developing the photoresist, an anisotropic etching ofthe insulator is carried out, resulting in the structure depicted inFIG. 4A. Preferably, wet etching of the insulator is performed, e.g., bydipping the substrate in an etching solution. This ensures that anundercut is developed in the insulating trenches, so as to prevent metaldeposition on the side walls. Metal deposition is carried out, resultingin the structure shown in FIG. 4B. Metal deposition is immediatelyfollowed by a lift off process to yield a structure as shown in FIG. 4C,which is now ready for the assembly of SWNTs, resulting in the completednanoswitch shown in FIG. 4D.

FIG. 5 depicts a sample layering, lithography, and etching process usedto build the etched substrate of a nanoswitch device according to theinvention. Substrate layer 10 can be any material that providesmechanical support for the device and can be fabricated with standardtechniques. The substrate is preferably a non-conducting (insulating) orsemi-conducting material such as silicon, silicon dioxide, or siliconnitride. If the substrate itself is not sufficiently insulating, theninsulating layer 20 can be added to the substrate, e.g., by thermaloxidation of silicon to form silicon dioxide. Preferably the insulatinglayer is at least 80 nm in thickness. The thickness of the insulatinglayer can be adjusted depending, for example, on the purity of thesubstrate material or the pattern of the trenches. The increasedpresence of impurities in, e.g., silicon used as the substrate base maybe compensated by using a thicker insulating layer. The preferredthickness of the insulating layer also increases as the width of thetrenches (first and second channels) increases. Suitable trenchdimensions that maintain the non-volatile nature of the nanoswitchstates can be determined, for example, by reference to FIG. 10A. Metalmask layer 22 is applied onto the insulating layer. For example, themetal mask layer can be a 20 nm thick layer of tungsten. Next,photoresist layer 24 is applied (e.g., spun on) to the mask layer. Forexample, the photoresist can be a 150 nm thick layer ofpolymethylmethacrylate (PMMA). Following application of the resist, thephotoresist is exposed to provide the pattern of conducting materialrequired to form the conductive terminals (electrodes) and channels(extension electrodes) of the device. For example, exposure can be bystandard electron beam lithography. After exposure of the resist, themetal mask layer is etched. For example, a tungsten mask layer can beetched using SF₆. The insulating layer is then etched. For example, CHF₃can be used to etch a silicon dioxide insulating layer to a depth ofabout 70 nm. An additional wet etching step (e.g., using a 1:50 dilutionof 30% HF) is preferred, so as to undercut the channel regions, whichwill inhibit metal deposition on the side walls; such metal depositioncould defeat the actuation mechanism. The preferred etch rate is about 4nm/min. Finally, the conductive layer is applied, and subsequently themask is lifted off. Any metallic conductor can be used for theconductive layer. For example, gold, silver, chromium, or anycombination thereof can be used. A preferred conducting layer is a 60 nmlayer of Cr/Au. Preferably, the conductive layer remains slightly belowthe surface of the adjacent insulating layer at the terminals(electrodes), and well below the insulating layer surface adjacent tothe channels (troughs).

According to the invention, assembly of one or more nanoelements such asSWNTs between the electrodes, for use as the actuator of the nanoswitch,is performed using dielectrophoresis. In a dielectrophoresis (DEP)process, it is well known that the dielectrophoretic force is determinedby the electrical properties of the medium and the object, the objectsize and shape, the geometry of the electrodes, and the frequency of theelectric field. The DEP force can be written as

$F_{DEP} = {\frac{\pi}{6}r^{2}l\; ɛ_{m}{Re}\left\{ {K(\omega)} \right\}{\nabla E_{r\; m\; s}^{2}}}$${K(\omega)} = \left( \frac{ɛ_{p}^{*} - ɛ_{m}^{*}}{ɛ_{m}^{*}} \right)$Where l and r are the length and radius of a rod-like particle, ε_(m) isthe real permittivity of the suspending medium, E_(rms) is the root meansquare (rms) of the electric field, and K(ω), is the Clausius-Mosottifactor. ε_(p)* and ε_(m)* are the complex permittivity of the rod-likeparticle and the suspending medium, respectively. ε*=ε−i(σ/ω), wherei=√{square root over (−1)}, ε is the real permittivity and σ is theconductivity of the material. E is a material property and can bewritten as a product of the relative permittivity of the material andpermittivity of free space ε₀.

In order to assemble one or more carbon nanotubes at actuator element ofthe nanoswitch by dielectrophoresis, the application of an AC voltagesource with a frequency in the range from about 100 kHz to about 10 MHzis preferred. A frequency of about 1 MHz is more preferred. Theamplitude of the AC voltage can be in the range from about 1 to about 5volts. Preferably, a phase shifter is used to provide voltage gradientsof opposite direction at the two electrodes during dielectrophoresis.Some adjustment of the voltage source conditions may be requireddepending on batch-to-batch differences in commercially suppliednanotubes.

The nanoelement or nanoelements used as the actuation mechanism for thenanoswitch are preferably either SWNT or multi-walled carbon nanotubes.The nanotube or nanotubes can be configured either as a bundlecontaining a small number of nanotubes, e.g., 1 or more, or about 1-10,2-10, 3-10, or 5-10 nanotubes, or as a single nanotube spanning thefirst and second terminals of the nanoswitch device. Single nanotubesare most preferred as the actuator. SWNT can be obtained from commercialsources as an aqueous suspension. For the studies described in theExamples, SWNT were obtained either from Nantero, Inc. (Woburn, Mass.)or from Brewer Science, Inc. (Rolla, Mo.). The SWNT suspensionsgenerally had a pH value in the range of 6-7 and a conductivity in therange from about 10 to about 40 μS/cm, or about 30 μS/cm. Whileindividual nanotubes are preferred as the actuator, if a greater numberof nanotubes is desired, then either the concentration of the nanotubesuspension can be increased, or the time of dielectrophoresis can beincreased.

The method of integrating a single nanowire, such as a SWNT or amulti-walled carbon nanotube, into the nanoswitch device of the presentinvention also can be used to integrate such a nanowire into ananocircuit, for example, a circuit or device that does not function asa switch but performs another function. A nanosubstrate forincorporation of the nanowire has a surface that contains at least twoexposed conductive terminals separated by an insulating area. A liquidsuspension of nanotubes, such as SWNT, is added to the surface of thenanosubstrate to cover the conductive terminals and a portion or all ofthe insulating area between them. Dielectrophoresis is then performed byapplying a voltage between the two terminals, for example, by using theterminals as electrodes for dielectrophoresis. Conditions fordielectrophoresis are similar to or the same as those for assembling aSWNT in the nanoswitch of the present invention. Followingdielectrophoresis, excess SWNT suspension can be removed using a streamof gas (e.g., nitrogen), such that only one SWNT remains in electricalcontact with both conductive terminals.

A nanoswitch device according to the invention is preferablynon-volatile and bistable, i.e., existing in one of two stable switchingstates for a long period of time (e.g., for at least an hour, a day, aweek, a month, a year, or longer) until a signal is sent to the deviceto switch it into the other state. In order to serve as a switch, in apreferred embodiment each state is characterized by possessing aconductance pathway between certain conductive terminals, but leaving anopen circuit between others. For example, a method of switching fromstate 1 to state 2 can include the steps of: (1) providing a bistable,non-volatile nanoswitch device in state 1, having a non-volatileelectrical connection between first and third terminals; (2) applying avoltage between second and fourth terminals; and (3) removing thevoltage between the second and fourth terminals. The method establishesa non-volatile electrical connection between the second and fourthterminals, and abolishes the electrical connection between the first andthird terminals. A related method of switching from state 2 to state 1can include the steps of: (1) providing a bistable, non-volatilenanoswitch device in state 2, having a non-volatile electricalconnection between second and fourth terminals; (2) applying a voltagebetween first and third terminals; and (3) removing the voltage betweenthe first and third terminals. The method establishes a non-volatileelectrical connection between the first and third terminals, andabolishes the electrical connection between the second and fourthterminals.

An advantage of the present nanoswitch is that approximately the samelow voltage will suffice to both switch the device from state 2 to state1 (first voltage) and to switch it from state 1 to state 2 (secondvoltage). The required voltage will depend on the particular design, buta switching voltage in the range of about 3 volts is preferred. Anyvoltage above the threshold required for switching can be used, butexcessive voltage that could damage the device should be avoided.

The following examples are presented to illustrate the advantages of thepresent invention and to assist one of ordinary skill in making andusing the invention. These examples are not intended in any wayotherwise to limit the scope of the disclosure.

EXAMPLES Example I Dielectrophoretic Assembly of SWNT Actuation Element

The assembly of SWNT from solution to form an actuation structure in ananoswitch device was carried out using a dielectrophoretic assemblyprocess. The process is illustrated schematically in FIG. 6.

An aqueous suspension of commercially available highly purified SWNTswas used in the experiment. The aqueous suspension was diluted to 10⁻⁵g/ml using deionized water, and 1 μL of SWNT suspension 110 wasdeposited onto the surface of etched, metal-coated substrate 100 so asto bridge and cover the insulating area between the first and secondelectrodes and containing the extension electrodes. The substrate wasconfigured for dielectrophoresis as shown in FIG. 6 (120) by attachingleads to the first and second electrodes, and attaching the electrodesto AC voltage source 130. Assembly was conducted at a frequency of 1 MHzwith the AC voltage between two electrodes in the range from 1 to 5 Vpp.After 1 min of assembly, the solution was dried using a stream ofnitrogen gas.

Example II Optimization of Electrode Configuration

In the experiment shown in Example I, the resulting nanotubes weredifficult to align appropriately and reproducibly by varying processparameters including pH, SWNT concentration, conductivity of thesolution, AC voltage, and frequency of the applied field. Therefore, thedielectrophoretic process was modified to achieve better assembly.Specifically, modifications were directed to the electric fieldgradient, represented as ∇E_(rms) ², in dielectrophoresis equations. Itwas discovered that a change of the electrode configuration wasnecessary to get the desired electrical field gradient.Dielectrophoresis was carried out using the same conditions as inExample I. In the case of flat electrodes, shown in FIG. 7A, a number ofSWNTs were assembled over a large area between the two electrodes,because flat electrodes provide a uniform electric field over a largearea. Electrodes with a triangular shape at their tips also werefabricated (FIG. 7B), and again used to assemble SWNT using the sameconditions as before. However, there were too many SWNTs assembledbetween two edge points as well as SWNT assembled at the angular point.Finally, very sharp electrodes were designed as shown in FIG. 7C andused to carry out the assembly of SWNTs using the same conditions. Usingthe sharp electrodes, the nanotubes assembled only between the tips, anddid not form between two edge points, because the strength of electricfield was reduced by the long distance between the edges.

Example III Use of Phase Shifting to Assemble SWNT Actuator

Using the sharp-tipped electrode configuration shown in FIG. 7C, asdescribed in Example II, the SWNTs were sometimes assembled on only oneelectrode. This could be explained by the fact that no electric fieldgradient existed on the ground electrode. Therefore, a phase shifter wasintroduced to produce a field gradient at the ground electrode withopposite phase to that of the phase electrode. This is depictedschematically in FIG. 8, where the phase electrode is represented aselectrode-A and the ground electrode as electrode-B. FIG. 8 alsopresents scanning electron microscopy (SEM) images of SWNTs assembledwithout (FIG. 8A) or with (FIG. 8B) the use of phase shifting. The useof the phase shifter led to uniform SWNT assembly at both electrodes.

Example IV Assembly of a Single-Stranded SWNT Actuator

Using the optimized dielectrophoresis conditions described in ExamplesI-III above (e.g., dielectrophoresis conditions of Example I, sharpelectrodes of FIG. 7C, use of phase shifting as in FIG. 8B), studieswere undertaken in order to achieve one single strand of SWNT assembledper device on the template. When the SWNT suspension was allowed to dryin ambient air following assembly by dielectrophoresis, invariably watermarks remained. These indicated that some residual SWNTs remained, whichcould result in a reduction of the yield of the device through thepresence of circuiting SWNTs in undesired positions. Hence an externalstream jet line of nitrogen gas was introduced to dry the solution veryefficiently, such that only one strand of SWNT is assembled. Nitrogen at15 psi was blown over the SWNT suspension droplet from a height of 1 mmfor 20 seconds. The process is depicted schematically in FIG. 9.Controlled assembly was effectively achieved with a high yield (about80-95% of devices having one SWNT as the actuator). An SEM micrograph ofa typical final nanoswitch device possessing a single-stranded SWNTactuator is shown in FIG. 9.

Example V Operational Characteristics of a Single-Stranded SWNT Actuator

A non-linear model was employed to determine the mechanical andelectrical limits for various device dimensions. Using the model, aphase diagram of the volatility of the bridge structure as a function ofthe gap between the electrode and the carbon nanotube and the width ofthe electrode was produced. The phase diagram is shown in FIG. 10A. Theupper limit of the actuation voltage is shown in FIG. 10B as a functionof the width and gap of the trench. Slippage of the SWNT at theSWNT-electrode interface was not considered.

While the present invention has been described in conjunction with apreferred embodiment, one of ordinary skill in the art, after readingthe foregoing specification, will be able to effect various changes,substitutions of equivalents and other alterations to the compositionsand articles set forth herein. It is therefore intended that theprotection granted by Letter Patent hereon be limited only by thedefinitions contained in the appended claims and equivalents thereof.

What is claimed is:
 1. A method of integrating a single nanowire into ananocircuit, the method comprising the steps of: providing ananosubstrate, a surface of the nanosubstrate comprising two conductiveterminals separated by an insulating area; adding a liquid suspension ofnanotubes to the surface of the nanosubstrate to cover the conductiveterminals; performing dielectrophoresis by applying a voltage betweenthe first and second conductive terminals; and removing excess nanotubesusing a stream of gas, whereby only one nanotube remains in electricalcontact with both conductive terminals.
 2. The method of claim 1,wherein the step of performing dielectrophoresis comprises applying anAC voltage in the range from about 1 to about 5 volts.
 3. The method ofclaim 2, wherein the AC voltage has a frequency of about 1 MHz.
 4. Themethod of claim 1, wherein dielectrophoresis is performed using a phaseshifter to provide an electrical field gradient at the ground electrodeof opposite phase to that at the phase electrode.
 5. The method of claim1, further comprising iteratively performing the method while adjustingthe concentration of nanotubes in the liquid suspension to identify aconcentration at which only one nanotube is attached to both conductiveterminals.
 6. The method of claim 1, further comprising iterativelyperforming the method while adjusting the duration of performingdielectrophoresis to identify a duration at which only one nanotube isattached to both conductive terminals.
 7. The method of claim 1, whereinthe liquid suspension comprises single-walled carbon nanotubes (SWNT)and the integrated single nanowire is an SWNT, or the liquid suspensioncomprises multi-walled carbon nanotubes (MWNT) and the integrated singlenanowire is an MWNT.
 8. The method of claim 1, wherein each of the firstand second conductive terminals comprises a pointed tip and theterminals are oriented with the tips pointing towards each other.
 9. Themethod of claim 8, wherein each tip has a profile which forms an acuteangle of about 30 degrees or less.
 10. The method of claim 1, whereinthe nanosubstrate comprises plural pairs of conductive terminals, eachpair separated by an insulating area, and a single nanowire isintegrated into said nanocircuit at each pair of conductive terminalssuch that each integrated nanowire is in electrical contact with a pairof conductive terminals.
 11. The method of claim 1, wherein theinsulating area forms an insulating gap separating the conductiveterminals, the gap being about 800 nm in length.
 12. The method of claim1, wherein the conductive terminals comprise a material selected fromgold, chromium, silver, and combinations thereof.
 13. The method ofclaim 1, wherein the insulating area comprises a material selected fromsilicon, silicon dioxide, and silicon nitride.
 14. The method of claim1, wherein the stream of gas comprises nitrogen.
 15. The method of claim1, wherein the stream of gas is pressurized at 15 psi, and the stream isblown over the liquid suspension on the surface of the nanosubstratefrom a height of about 1 mm above the nanosubstrate for a duration ofabout 20 seconds.